Disk drive track address encoded with a servo sector address

ABSTRACT

A servo sector address and a track address of a recording medium of a disk drive are encoded into a combined address value. The combined address value is stored in a combined address field that has fewer bits than the total bits required to uniquely encode the servo sector address and the track address. The position of a transducer head, indicated by the servo sector address and the track address, is determined by reading encoded values from two consecutive servo sectors on the recording medium and then decoding the encoded values.

BACKGROUND OF THE INVENTION

1. Field of the Invention

Embodiments of the present invention relate generally to disk drivesand, more particularly, to systems and methods for encoding track andservo sector addresses in such drives.

2. Description of the Related Art

A disk drive is a data storage device that stores digital data inconcentric tracks on the surface of a data storage disk. The datastorage disk is a rotatable hard disk with a layer of magnetic materialthereon, and data is read from or written to a desired track on the datastorage disk using a transducer head, i.e., a read/write head, that isheld proximate to the track while the disk spins about its center at aconstant angular velocity.

To properly align the read/write head with a desired track during a reador write operation, disk drives generally use a closed-loop servo systemthat relies on servo data stored in servo sectors written on the disksurface when the disk drive is manufactured. These servo sectors form“servo wedges” or “servo spokes” from the outer to inner diameter of thedisk, and are either written on the disk surface by an external device,such as a servo track writer, or by the drive itself using a selfservo-writing procedure. The read/write head can be positioned withrespect to the data storage disk by using feedback control based onservo information read from the servo wedges with the read element ofthe read/write head. The servo sectors provide position informationabout the radial and circumferential location of the read/write headwith respect to the disk surface in the form of servo patterns.

A typical servo pattern consists of a preamble field used to synchronizethe timing of the read channel and to adjust the signal amplitude, anarea that provides track number and servo sector number used for coarsepositioning of the read/write head, and servo bursts for finepositioning of the read/write head relative to a specific data storagetrack. The pattern typically includes a field for enumerating each servosector and a field for enumerating each data storage track, where theservo sector number provides the circumferential position of theread/write head and the track number provides the radial position of theread/write head.

Due to the large number of servo sectors commonly found on a datastorage disk, i.e., several hundreds, and the very large number oftracks found on a data storage disk, i.e., hundreds of thousands, fieldsenumerating the servo sector and track numbers occupy a significantportion of storage space on the disk that could otherwise be dedicatedfor user data. Because the information in these fields is largelyredundant, with most bits remaining unchanged from track to track andfrom sector to sector, schemes have been developed in the art forminimizing the size of these fields

In some schemes, the field for servo sector number is eliminatedentirely and is replaced by an index bit that serves as an index mark todesignate a particular servo sector for each track. The index bit is setto an index value, e.g., 1, for one servo sector per track, the indexbit being set to a null value, e.g., 0, for all other servo sectors onthe track. A significant drawback to this approach is that, when theposition the of the read/write head is unknown, the time required toreestablish a known position of the read/write head is, on average, onehalf revolution of the disk. Because head location is frequently unknownduring normal operation of a disk drive, for example whenever switchingbetween surfaces of a drive, this is a significant drawback.

Other schemes combine the track address and servo sector address into asingle smaller field. In some cases such schemes introduce ambiguitiesin the actual position of the read/write head, since a portion of theunique address information is sacrificed in order to reduce the size ofthe track address/servo sector address field. Positional ambiguity ishighly undesirable in the context of reliability of disk driveperformance. In other cases, such schemes have the added constraint thatthe position of the read/write head can only be determined when it ispositioned on the same track for some number of servo sectors, whichalso increases the time necessary to determine head position.

In light of the above, there is a need in the art for a system andmethod of encoding track addresses and servo sector addresses so that aminimal portion of a disk drive storage medium is used while allowinghead position to be quickly determined.

SUMMARY OF THE INVENTION

One or more embodiments of the present invention provide a system andmethod for encoding servo sector and track addresses of a disk driverecording medium together into a combined address value. The combinedaddress value is stored in a combined address field that has fewer bitsthan the total bits required to uniquely encode the servo sector addressand the track address. The position of a transducer head, indicated bythe servo sector address and the track address, is determined by readingencoded values from two consecutive servo sectors on the recordingmedium and then decoding the encoded values.

A method of decoding a sector number and a track number of a recordingmedium of a disk drive based on encoded data written on servo sectors ofthe recording medium, according to an embodiment of the presentinvention, comprises the steps of consecutively reading first and secondencoded data, the first encoded data from a first servo sector and thesecond encoded data from a second servo sector, decoding an S-bit sectornumber from the first and second encoded data, and decoding an L-bittrack number from the first and second encoded data, wherein each of thefirst and second encoded data contains less than (S+L) bits.

A non-transitory computer-readable storage medium, according to anembodiment of the present invention, comprises instructions for causinga controller of a disk drive to carry out the steps of consecutivelyreading first and second encoded data written on servo sectors of arecording medium of the disk drive, the first encoded data from a firstservo sector and the second encoded data from a second servo sector,decoding an S-bit sector number from the first and second encoded data,and decoding an L-bit track number from the first and second encodeddata, wherein each of the first and second encoded data contains lessthan (S+L) bits.

A recording medium for a disk drive, according to an embodiment of thepresent invention, comprises a plurality of servo sectors each having aplurality of address fields, one for each track of the recording medium,wherein each address field contains an L-bit encoded data, the lowerbits of which match lower bits of a track number of the trackcorresponding to the address field. The upper bits of the L-bit encodeddata, however, do not match the upper bits of the track number.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the above recited features of the presentinvention can be understood in detail, a more particular description ofthe invention, briefly summarized above, may be had by reference toembodiments, some of which are illustrated in the appended drawings. Itis to be noted, however, that the appended drawings illustrate onlytypical embodiments of this invention and are therefore not to beconsidered limiting of its scope, for the invention may admit to otherequally effective embodiments.

FIG. 1 is a perspective view of an exemplary embodiment of a disk drive.

FIG. 2 illustrates a storage disk according to embodiments of theinvention.

FIG. 3 is a schematic diagram of a servo sector disposed in a servowedge, according to embodiments of the invention.

FIG. 4 schematically illustrates a comparison of a combined addressfield with a typical servo sector address field and a typical trackaddress field.

FIG. 5 is a block diagram conceptually illustrating the manner in whicha combined address value is constructed from a servo sector addressvalue and a track address value, according to embodiments of theinvention.

FIG. 6 is a flow chart that summarizes, in a stepwise fashion, a methodfor determining a position of a transducer head relative to a storagedisk, according to an embodiment of the invention.

For clarity, identical reference numbers have been used, whereapplicable, to designate identical elements that are common betweenfigures. It is contemplated that features of one embodiment may beincorporated in other embodiments without further recitation.

DETAILED DESCRIPTION

FIG. 1 is a perspective view of an exemplary embodiment of a disk drive110. For clarity, disk drive 110 is illustrated without a top cover.Disk drive 110 includes a storage disk 112 that is rotated by a spindlemotor 114. Spindle motor 114 is mounted on a base plate 116. An actuatorarm assembly 118 is also mounted on base plate 116, and has a slider 120mounted on a flexure arm 122 with a read/write head 127. Flexure arm 122is attached to an actuator arm 124 that rotates about a bearing assembly126. Voice coil motor 128 moves slider 120 relative to storage disk 112,thereby positioning read/write head 127 over the desired concentric datastorage track disposed on the surface 112A of storage disk 112. Spindlemotor 114, read/write head 127, and voice coil motor 128 are coupled toelectronic circuits 130, which are mounted on a printed circuit board132. The electronic circuits 130 include a read channel, amicroprocessor-based controller, and random access memory (RAM). Forclarity of description, disk drive 110 is illustrated with a singlestorage disk 112 and actuator arm assembly 118. Disk drive 110 may alsoinclude multiple storage disks 112 and multiple actuator arm assemblies118. In addition, each side of disk 112 may have an associatedread/write head 127 coupled a flexure arm 122. The invention describedherein is equally applicable to devices wherein the individual heads areconfigured to move separately some small distance relative to theactuator using dual-stage actuation.

FIG. 2 illustrates storage disk 112 with data organized in a typicalmanner after servo wedges 244 have been written on storage disk 112 byeither a media writer or by disk drive 110 itself via self servo-write(SSW). Storage disk 112 includes concentric data storage tracks 242located in data sectors 246 for storing data. Concentric data storagetracks 242 are positionally defined by servo information written inservo wedges 244. Each of concentric data storage tracks 242 isschematically illustrated as a centerline, but in practice occupies afinite width about a corresponding centerline. Substantially radiallyaligned servo wedges 244 are shown crossing concentric data storagetracks 242 and include servo sectors containing servo information thatdefines the radial position and track pitch, i.e., spacing, ofconcentric data storage tracks 242. Such servo information includes aservo sector containing a reference signal that is read by read/writehead 127 during read and write operations to position read/write head127 above a desired track 242. Servo sectors are described in greaterdetail in conjunction with FIG. 3, below. In practice, servo wedges 244may be somewhat curved, for example, configured in a spiral pattern.Typically, the actual number of concentric data storage tracks 242 andservo wedges 244 included on storage disk 112 is considerably largerthan illustrated in FIG. 2. For example, storage disk 112 may includehundreds of thousands of concentric data storage tracks 242 and hundredsof servo wedges 244.

When disk drive 110 is in operation, actuator arm assembly 118 sweeps anarc between an inner diameter (ID) and an outer diameter (OD) of storagedisk 112. Actuator arm assembly 118 accelerates in one angular directionwhen current is passed through the voice coil of voice coil motor 128and accelerates in an opposite direction when the current is reversed,allowing for control of the position of actuator arm assembly 118 andattached read/write head 127 with respect to storage disk 112. Voicecoil motor 128 is coupled with a servo system known in the art that usespositioning data read from storage disk 112 by read/write head 127 todetermine the position of read/write head 127 over concentric datastorage tracks 242. The servo system determines an appropriate currentto drive through the voice coil of voice coil motor 128, and drives saidcurrent using a current driver and associated circuitry.

FIG. 3 is a schematic diagram of a servo sector 300 disposed in servowedges 244, according to embodiments of the invention. Servo sector 300is an exemplary illustration of servo sector formatting, and includes apreamble field 301, a servo mark field 302, a combined address field350, a pad field 303, and servo burst fields 304A-304D. It is noted thatthe format of servo sector 300 is an exemplary illustration and thespecific order of the different fields making up servo sector 300 mayvary from the format described in FIG. 3 without departing from thescope of the invention. Preamble field 301 synchronizes the timing ofthe read channel and adjusts signal amplitude, servo mark field 302includes information indicating a beginning of the servo area, and padfield 303 is included as a buffer between combined address field 350 andservo burst fields 304A-304D. Servo burst fields 304A-304D contain finehead positioning information for locating concentric data storage tracks242. Several methods are well known in the art for implementing suchfine positioning, including amplitude servo bursts or phase-encodedservo bursts, the method used depending on the demodulation system.

Combined address field 350 includes the servo sector address for servosector 300 and the track address of the track on which servo sector 300is disposed. According to embodiments of the invention, this informationis combined into a single value, i.e., a combined address value, andstored in a field having fewer bits than the total bits in a servosector address field sized for storing the servo sector address and atrack address field sized for storing the track address. The servosector address and the track address are reversibly combined into acombined address value in a way that allows the servo sector address andthe track address to be unambiguously extracted by reading the combinedaddress value for two locations on a disk, where the two locations arenot necessarily on the same track. Thus, the position of read/write head127 can be determined by reading the combined address value for twolocations on a disk, which is significantly faster than waiting forstorage disk 112 to rotate to the index bit.

FIG. 4 schematically illustrates a comparison of combined address field350 with a typical servo sector address field 401 and a typical trackaddress field 402. Servo sector address field 401 is a servo sectorfield sized to include the bit values for a servo sector address onstorage disk 112. Since there are typically several hundred servosectors in a disk drive, the bit length 410 of servo sector addressfield 401 is nine bits, which can accommodate 511 unique servo sectoraddresses in the preferred embodiment of this invention. Similarly,track address field 402 is a servo sector field that is sized to includethe bit values for a track address on storage disk 112, and has a bitlength 420 of 18 bits. As shown, the number of bits in combined addressfield 350 is substantially smaller than the sum of bit length 410 andbit length 420, thereby substantially reducing the size of servo sector300 and consequently the portion of storage disk 112 occupied by servowedges 244. In some embodiments, combined address field 350 may have abit length as small as bit length 410 of track address field 402.

FIG. 5 is a block diagram conceptually illustrating the manner in whicha combined address value 510 is constructed from a servo sector addressvalue 501 and a track address value 502, according to embodiments of theinvention. Servo sector address value 501 is the address value for aparticular servo sector on storage disk 112 and has a bit length equalto bit length 410 of servo sector address field 401, e.g., 9 bits. Inthe embodiment illustrated in FIG. 5, servo sector address value 501 hasa bit length of S bits, where S is defined below in conjunction withEquation 1. Track address value 502 is the address value for aparticular track on storage disk 112 and has a bit length equal to bitlength 420 of track address field 402, e.g., 18 bits. In the embodimentillustrated in FIG. 5, track address value 502 has a bit length of Lbits, where L is defined below in conjunction with Equation 3. In theexemplary embodiment described herein, combined address value 510 alsohas a bit length equal to bit length L. In other embodiments, combinedaddress value 510 may have a bit length greater than bit length 420 aslong as it is less than the total bit length of servo sector addressfield 401 and the track address field 402, without departing from thescope of the invention.

Initially, a “munged” sector number m(s) is computed from servo sectoraddress value 501, as indicated by munge operation 520 in FIG. 5.Equation 1 explicitly describes munge operation 520 on servo sectoraddress value 501, herein denoted s:

$\begin{matrix}{{m(s)} = {{mod}( {\frac{( {s + {2\; c}} )( {s + 1} )}{2},2^{s}} )}} & (1)\end{matrix}$where S is an integer large enough to satisfy the condition s<2^(S) forall s, and c is a constant non-negative integer, described below. Thus,S may be equal to the number of servo sector bits, bit length 410. Theterm “munging,” as used herein and applied to the operation described byEquation 1, means that information is being (imperfectly) transformed,and is not to be confused with the meaning of the term “munge” relatedto constructing a strong password through character substitution. Inaddition, the notation mod(a, b) used for the modulo operation herein issimply a convenient alternative to the notation “a modulo b” or “a modb.”

It is noted that Equation 1 defines m(s) so that Equation 2 is true forall s>0:mod [m(s)−m(s−1)−c,2^(S) ]=s  (2)

Thus for all s>0, the difference (i.e., modulo [2^(S)]) betweenconsecutive munged sector numbers, i.e., m(s) and m(s−1), determines thesector number, and s can almost always be determined from twoconsecutive sector readings. The exceptional case is where s=0. Assumingthat there are N servo sectors, numbered 0 to N−1, there is always aconstant non-negative integer c, such that mod [m(0)−m(N−1)−c, 2^(S)] is≧N or 0. Thus, it is always possible to uniquely determine the sectornumber from the backward difference of consecutive munged sector numbersm(s) even when the sector number “wraps” from N−1 to 0. This property isan improvement over other known and patented methods.

Embodiments of the invention combine munged sector number m(s) and trackaddress value 502 to produce combined address value 510 in a way thatpreserves the useful property of the backward difference illustrated byEquation 2. Equation 3 expresses the formula for computing combinedaddress value 510, herein denoted tau(t,s), from munged sector numberm(s) and track address value 502, herein denoted t:tau(t,s)=mod(t+2^((L-S)) *m(s),2^(L))  (3)where L≧S. In a preferred embodiment, L is an integer that equals thenumber of track address bits, i.e., bit length 420, but may also be alarger integer. Application of Equation 3 places the munged sectornumber m(s) in the upper bits of track address value 502, as denoted bybit-shift operation 530 and modulo addition operation 540 in FIG. 5,thereby leaving the lower bits of track address value 502, i.e., lowerbit region 502A, uncorrupted. In a preferred embodiment, the resultingcombined address is Gray coded and written to the media.

The definition of combined address value 510 as expressed by Equation 3has several properties that allow servo sector address value 501 andtrack address value 502 to be determined when combined address value 510is read from two consecutive sectors. Given a coarse estimate of theradial velocity of read/write head 127, the two consecutive sectors arenot required to be on the same track. Any of well-known methods forestimating the radial velocity of read/write head 127 may be used. Anestimated velocity of zero tracks per servo sector is usually sufficientsince the radial velocity during a head switch is typically quite low.For an initial head load, the radial velocity is also typically low anda zero velocity estimate is usually sufficient. The back electro-motiveforce from the voice coil can provide a more refined estimate of theradial velocity. If it becomes necessary to reacquire the track addressand sector number address during a high velocity seek, a typicalimplementation provides an estimate of the last known radial velocity.

A description is now provided for determining both track address value502 and servo sector address value 501 from two consecutive observationsof combined address value 510, i.e., tau(t2,s) and tau(t1,s−1), from twounknown tracks herein denoted t1 and t2 tau(t2,s) and tau(t1,s−1) may beobservations from different concentric data storage tracks 242 onstorage disk 112, assuming that an estimated radial velocity v at whichread/write head 127 has moved between the first and second tracks, i.e.,t1, t2, is within an acceptable range of the actual radial velocity.Specifically, if estimated radial velocity v (expressed intracks/sector) is within −2^((L-S-1))+1 and 2^((L-S-1)) of the actualradial velocity at which read/write head 127 has moved between the firstand second tracks, then the computed track address value 502 and servosector address value 501 will be correct. Considering that in typicalembodiments L=18 and S=9, estimated radial velocity v is typicallyrequired to be within ±256 tracks/sector of the actual radial velocityof read/write head 127, which is not a difficult condition to meet. Oneof skill in the art will appreciate that producing a radial velocityestimate of this accuracy can be readily accomplished by a variety ofmeans. For example, in one embodiment, a back electro-motive force fromvoice coil motor 128 turning inside the magnet can provide such anapproximation. Thus, assuming that radial velocity v is correct, anexpected combined address value 510, i.e., tau_(exp), for cylinder t2and sector s−1 can be expressed by Equation 4:tau _(exp)(t2,s−1)=mod [smod(tau(t2,s)−(tau(t1,s−1)+v),2^((L-S)))+(tau(t1,s−1)+v),2^(L)]  (4)where smod is the “signed modulo” operator. The signed modulo operatoris defined by Equation 5:

$\begin{matrix}{{s\;{{mod}( {x,N} )}} = {{{mod}( {{x + \frac{N}{2}},N} )} - \frac{N}{2}}} & (5)\end{matrix}$and produces a value in the range −N/2, N/2−1 inclusive for integral xand even integral N. Thus, the signed modulo operator is substantiallysimilar to the modulo operator, with the addition of an offset thatprovides both positive and negative differences.

Given a combined address value 510 from one track and an estimate of thecombined address for that same track on the previous servo sector, theservo sector address value 501 can be determined with Equation 6:

$\begin{matrix}{s = \frac{ {{{mod}\lbrack {{{tau}( {{t\; 2},s} )} - {{tau}_{\exp}( {{t\; 2},{s - 1}} )} - {c\; 2^{({L - S})}}} )},2^{L}} \rbrack}{2^{({L - S})}}} & (6)\end{matrix}$Track address value 502, i.e., t2, is computed using Equation 7:t2=mod [tau(t2,s)−2^((L-S)) m(s),2^(L)]  (7)Thus, both servo sector address value 501 and track address value 502can be calculated from two consecutive observations of combined addressvalue 510.

As noted above, the inventor has determined that there is always aconstant non-negative integer c available, such that mod [m(0)−m(N−1)−c,2^(S)] is ≧N or 0, where N is the number of sectors on a surface ofstorage disk 112. The constant c makes it possible to uniquely determineservo sector address value 501, i.e., s, from the backward difference oftwo consecutive munged sector number m(s), even when the sector number“wraps” from N−1 to 0. Equation 8 calculates one possible value for c:

$\begin{matrix}{c = \lfloor \frac{{mod}( {\frac{n( {n - 1} )}{2},2^{S}} )}{2^{S} - N} \rfloor} & (8)\end{matrix}$where N is the number of sectors and S is the number of bits used torepresent the sector number. Table 1 provides sample output fromEquation 8 for N near 400, including the value of c computed andverification that the sector number formula works correctly at the wrappoint from N−1 to 0.

TABLE 1 No. Of Sector Number No. of Sector at Wrap Sec- Number mod(m(0)− Verifi- tors, N Bits, S c m(0) m(N − 1) m(N − 1) − c, 2{circumflexover ( )}S) cation 397 9 2 2 40 472 TRUE 398 9 1 1 41 471 TRUE 399 9 0 041 471 TRUE 400 9 3 3 104 408 TRUE 401 9 2 2 106 406 TRUE 402 9 1 1 107405 TRUE 403 9 0 0 107 405 TRUE 404 9 4 4 78 434 TRUE 405 9 3 3 81 431TRUE 406 9 2 2 83 429 TRUE 407 9 1 1 84 428 TRUE

FIG. 6 is a flow chart that summarizes, in a stepwise fashion, a method600 for determining a position of a transducer head relative to astorage disk, according to an embodiment of the invention. Method 600 isdescribed in terms of a disk drive substantially similar to disk drive110 in FIG. 1. However, other disk drives may also benefit from the useof method 600. The commands for carrying out steps 601-605 may reside inthe disk drive control algorithm and/or as values stored in theelectronic circuits of the disk drive or on the storage disk itself.

In step 601, the combined address value 510 is observed for twoconsecutive sectors on storage disk 112, i.e., tau(t2, s) and tau(t1,s−1). As noted above, each sector may be disposed on a different trackof storage disk 112.

In step 602, an expected combined address value 510, i.e., tau_(exp), iscomputed using Equation 4 and an estimate of the radial velocity ofread/write head 127 as it traveled between the two sectors during step601.

In step 603, given the expected combined address value 510 computed instep 602, i.e., tau_(exp), Equations 6A, 6B, can be used to determineservo sector address value 501 for one of the two sectors observed instep 601, i.e., sector (t2, s).

In step 604, given servo sector address value 501 determined in step603, Equation 7 is used to compute track address value 502 for thesector (t2, s).

By way of validation, Table 2 provides sample output of method 600,including decoded sector and track numbers. For generating Table 2, thenumber of cylinder bits L was 19, the number of sector bits S was 9, thenumber of sectors N was 400, the value of c used was 3, the actualvelocity of the read head was 550 tracks/sector, and the estimatedradial velocity was 250 tracks/sector. Because the wrap point betweensector numbers 399 and 0 is of particular interest with respect toembodiments of the invention, results for sectors 391-13 are illustratedin Table 2. As shown, the decoded track numbers match the actual tracknumbers and the decoded sector numbers match the actual sector numbers.

TABLE 2 Encoding Decoding munged munged expected munged actual actualsector cylinder/sector cylinder/sector decoded decoded track sectornumber number number decoded munged track # # m(s) tau(t, s) tau_exp(t2,s − 1) sector # sector # # 1000 391 500 513000 1550 392 383 393742513550 392 383 1550 2100 393 267 275508 394292 393 267 2100 2650 394 152158298 276058 394 152 2650 3200 395 38 42112 158848 395 38 3200 3750 396437 451238 42662 396 437 3750 4300 397 325 337100 451788 397 325 43004850 398 214 223986 337650 398 214 4850 5400 399 104 111896 224536 399104 5400 5950 0 3 9022 112446 0 3 5950 6500 1 7 13668 9572 1 7 6500 70502 12 19338 14218 2 12 7050 7600 3 18 26032 19888 3 18 7600 8150 4 2533750 26582 4 25 8150 8700 5 33 42492 34300 5 33 8700 9250 6 42 5225843042 6 42 9250 9800 7 52 63048 52808 7 52 9800 10350 8 63 74862 63598 863 10350 10900 9 75 87700 75412 9 75 10900 11450 10 88 101562 88250 1088 11450 12000 11 102 116448 102112 11 102 12000 12550 12 117 132358116998 12 117 12550 13100 13 133 149292 132908 13 133 13100

In sum, embodiments of the invention have the advantage of compressing aservo sector address field and a track address field into a combinedaddress field that is substantially smaller than the two combinedfields. Because the combined address field can be decoded by observingtwo consecutive sectors disposed on different tracks, the process ofdetermining the position of a read/write head in a disk drive can beperformed rapidly. In addition, because the formulas used to decode thecombined address field according to embodiments of the inventionprincipally involve multiplication and division by 2 and modulo andsigned modulo operators with power of 2 modulus, solving such formulasis straightforward using binary computers. For example, multiplicationand division by 2 simply require left and right shifts respectively, andthe modulo operator with a power of 2 modulus requires a pair of logicalor arithmetic shifts.

While the foregoing is directed to embodiments of the present invention,other and further embodiments of the invention may be devised withoutdeparting from the basic scope thereof, and the scope thereof isdetermined by the claims that follow.

1. A method of decoding a sector number and a track number of arecording medium of a disk drive based on encoded data written on servosectors of the recording medium, comprising the steps of: consecutivelyreading first and second encoded data, the first encoded data from afirst servo sector and the second encoded data from a second servosector; decoding an S-bit sector number from the first and secondencoded data; and decoding an L-bit track number from the first andsecond encoded data, wherein each of the first and second encoded datacontains less than (S+L) bits.
 2. The method of claim 1, wherein firstand second encoded data are read using a transducer head of the diskdrive and the decoding of the S-bit sector number and the L-bit tracknumber is also based on an estimated radial velocity of the transducerhead at the time of the reading of the first and second encoded data. 3.The method of claim 2, wherein each of the first and second encoded datacontains L bits.
 4. The method of claim 3, wherein the first servosector and the second servo sector are located on the same track.
 5. Themethod of claim 3, wherein the first servo sector and the second servosector are located on different tracks.
 6. The method of claim 3,wherein the recording medium has N distinct servo sectors and the firstservo sector represents the (N−1)-th servo sector and the second servosector represents the 0-th servo sector.
 7. The method of claim 6,wherein the first servo sector and the second servo sector are locatedon different tracks.
 8. A non-transitory computer-readable storagemedium comprising instructions for causing a controller of a disk driveto carry out the steps of: consecutively reading first and secondencoded data written on servo sectors of a recording medium of the diskdrive, the first encoded data from a first servo sector and the secondencoded data from a second servo sector; decoding an S-bit sector numberfrom the first and second encoded data; and decoding an L-bit tracknumber from the first and second encoded data, wherein each of the firstand second encoded data contains less than (S+L) bits.
 9. Thenon-transitory computer-readable storage medium of claim 8, whereinfirst and second encoded data are read using a transducer head of thedisk drive and the decoding of the S-bit sector number and the L-bittrack number is also based on an estimated radial velocity of thetransducer head at the time of the reading of the first and secondencoded data.
 10. The non-transitory computer-readable storage medium ofclaim 9, wherein each of the first and second encoded data contains Lbits.
 11. The non-transitory computer-readable storage medium of claim10, wherein the first servo sector and the second servo sector arelocated on the same track.
 12. The non-transitory computer-readablestorage medium of claim 10, wherein the first servo sector and thesecond servo sector are located on different tracks.
 13. Thenon-transitory computer-readable storage medium of claim 10, wherein therecording medium has N distinct servo sectors and the first servo sectorrepresents the (N−1)-th servo sector and the second servo sectorrepresents the 0-th servo sector.
 14. The non-transitorycomputer-readable storage medium of claim 13, wherein the first servosector and the second servo sector are located on different tracks. 15.A disk drive that decodes a sector number and a track number of arecording medium based on encoded data written on servo sectors of therecording medium, comprising: a transducer head configured toconsecutively read first and second encoded data, the first encoded datafrom a first servo sector and the second encoded data from a secondservo sector; a first decoder configured to decode an S-bit sectornumber from the first and second encoded data; and a second decoderconfigured to decode an L-bit track number from the first and secondencoded data, wherein each of the first and second encoded data containsless than (S+L) bits.
 16. The disk drive of claim 15, wherein the firstdecoder is configured to decode the S-bit sector number based on anestimated radial velocity of the transducer head at the time of thereading of the first encoded data, and the second decoder is configuredto decode the L-bit sector number based on an estimated radial velocityof the transducer head at the time of the reading of the second encodeddata.
 17. The disk drive of claim 16, wherein each of the first andsecond encoded data contains L bits.
 18. The disk drive of claim 17,wherein the first servo sector and the second servo sector are locatedon the same track.
 19. The disk drive of claim 17, wherein the firstservo sector and the second servo sector are located on differenttracks.
 20. The disk drive of claim 17, wherein the recording medium hasN distinct servo sectors and the first servo sector represents the(N−1)-th servo sector and the second servo sector represents the 0-thservo sector.